1. Field of the Invention
This invention relates to a process of producing crystalline D-sorbitol through evaporation of an aqueous D-sorbitol solution in a vacuum and subsequent melt crystallization of the D-sorbitol melt obtained during the evaporation.
2. Prior Art
D-sorbitol belongs to the hexahydric alcohols and has the empirical formula C.sub.6 H.sub.14 O.sub.6. D-sorbitol is made from D-glucose through electrolytic reduction or catalytic hydrogenation. D-sorbitol has a sweet taste and is used as sugar substitute, and it is also used in the cosmetic and pharmaceutical industries and for the production of polyethers and surfactants. D-sorbitol crystallizes in the .alpha.-, .beta.- and .gamma.-modification, where the .gamma.-modification is the thermodynamically stable form. D-sorbitol is used either in the form of concentrated aqueous solutions or in solid, crystalline form as industrial raw material. To ensure an easy handling of the solid crystalline D-sorbitol, it is desired that it should largely be present in the .gamma.-modification, as this form is not only thermodynamically stable, but also comparatively little hygroscopic.
From the DE-A 32 45 170 there is known a process of producing sorbitol with improved pelletizing properties, where a solution of crystallized glucose hydrogenates at a temperature below 170.degree. C., and the sorbitol solution thus obtained is spray-dried at a temperature in the range from 140 to 170.degree. C., so that the product obtained has a water content of less than 1%.
From the DE-C 23 50 619 there is known a process of continuously producing crystallized sorbitol by introducing molten and granular sorbitol in a container, in which the mass thus obtained is kept in movement at an elevated temperature. In this process, atomized sorbitol or sorbitol in the form of droplets, sheets, mists or strips with more than 90% dry matter is continuously introduced in the container in the molten condition together with 20 to 80 wt-% sorbitol powder with a grain size below 5 mm, and the mass thus obtained is agitated by rotating the open container with a horizontal axis or an axis inclined towards the horizontal, where molten sorbitol or sorbitol powder is applied onto the surface of the agitated mass. The agitated mass is maintained at a temperature of more than 90.degree. C., and the larger grains precipitating chiefly on the surface of the agitated mass are trapped at the outlet of the rotating container due to overflow and are subsequently subjected to a maturing stage for crystallizing the sorbitol. Before the melt crystallization there is provided a vessel for the vacuum evaporation, which is suited to bring a sorbitol solution to a dry content of more than 98 wt-%.
The DE-A 37 32 141 outlines the known processes of producing solid sorbitol. In one of these processes the sorbitol solution is evaporated in a vacuum until an almost anhydrous melt is obtained. Upon cooling, possibly by adding crystalline sorbitol, the melt solidifies, and from the solidified melt granular dust-like sorbitol is obtained by crushing and grinding. However, the product thus produced has a very wide range of particle sizes, and the content of .gamma.-sorbitol is subject to large fluctuations. In addition to crystalline sorbitol, considerable amounts of amorphous sorbitol are contained. This results in poor free-flowing properties, and during storage the product agglomerates to form a hard mass. In accordance with a further known process, a concentrated sorbitol solution is dripped or coarsely sprayed onto a mechanically moving bed of crystallized sorbitol. The moving and wetted crystal bed is maintained at a temperature of 60 to 80.degree. C. for vaporizing the water by passing air or an inert gas therethrough. After a corresponding crystallization period, part of the solid sorbitol is continuously withdrawn while the same is cooled. The product thus obtained likewise has a very wide range of particle sizes. It is also disadvantageous that the individual sorbitol particles quickly agglomerate to form hard masses as they absorb little moisture from the air, and that the dissolution period in water is relatively long. Finally, there is known a further process, where a 50 to 80% purified sorbitol solution is continuously split into extremely fine droplets by means of a spraying device, and at the same time finely divided crystalline sorbitol together with air is introduced in a spray tower such that the sprayed sorbitol solution covers the crystalline sorbitol particles with a thin film. For the vaporization of water there is additionally blown in a stream of hot air. The residual moisture can be varied within relatively wide limits by the temperature of the hot air and the ratio between the hot air and the sorbitol solution. After a crystallization period of 20 to 90 minutes, the dry product is cooled to a temperature &lt;40.degree. C., and part of the product is withdrawn, while the other part is recirculated to the spray tower. Even the handling properties of the crystalline sorbitol produced in accordance with this process are not satisfactory in the final analysis. To eliminate the disadvantages of the products produced in accordance with the known processes, the DE-A 37 32 141 proposes a process of producing crystalline sorbitol with improved handling properties, which operates according to the principle of spray drying, where a 50 to 80% aqueous sorbitol solution and crystalline sorbitol are supplied to the spray drier.